1. Field of the Invention
The present invention relates to a laser scanning apparatus adapted to use in an image forming apparatus such as an electro-photograph type printer, a laser facsimile machine, a digital copier, etc., which images light fluxes such as laser beams or light beams emitted from a light source on an imaging surface of a photosensitive element, and more particularly, to a laser scanning apparatus having an imaging optical system in which only one f-theta lens is used to image light fluxes reflected from a light deflector as uniform light spots on the imaging surface of the photosensitive element.
2. Description of the Related Art
Generally, a laser scanning apparatus which is used in an image forming apparatus such as a laser printer, a digital copier, a laser facsimile machine, etc., uses a light source to generate light fluxes such as laser beams to form an electrostatic latent image on a photosensitive element such as a photosensitive drum according to an image signal.
The laser scanning apparatus has a first imaging optical system to magnify or reduce light fluxes emitted from the light source, into parallel-linear lights and to lead them to a light deflector rotating in a high speed in a given direction. The laser scanning apparatus also has a second imaging optical system to converge light fluxes reflected from the light deflector as uniform light spots on an imaging surface of the photosensitive element to form the electrostatic latent image.
The first and second imaging optical systems generally include more than two lenses or mirrors. Particularly, since the second imaging optical system has expensive large-sized f-theta lenses of glass or plastic material or a combination of plural f-theta lenses and/or reflection mirrors, there is a problem that the lenses and/or mirrors are very heavy and miniaturization of their size is difficult. Recently, the second imaging optical system combined by two f-theta lenses and/or mirrors has been designed and used to obtain good optical performance by minimizing their size and cutting down their expense.
Referring now to FIG. 1, there is illustrated a general light scanning apparatus 10 to form an electrostatic latent image on an imaging surface of a photosensitive element.
The light scanning apparatus 10 includes a semiconductor laser 20 to emit laser beams according to an image signal mounted on a printed circuit board disposed on a main body frame 14, a collimating lens 22 to change the laser beams emitted from the semiconductor laser 20 into lights parallel to an optical axis, and an opening part 24 to standardize the laser beams penetrating through the collimating lens 22 in a required size. The light scanning apparatus also includes a cylindrical lens 26 to converge the laser beams to a sub-scanning direction and to change laser beams into linear lights in a main scanning direction, and a light deflector 28 disposed at a position converging the laser beams in the form of linear lights emitted through the cylindrical lens 26 to deflect a reflective direction of the laser beams.
The light deflector 28 is provided with a polygon mirror 29 having a plurality of deflective reflection surfaces 29a to deflect the reflective direction of the laser beams by moving the laser beams at a constant linear velocity. The light detector 28 is also provided with a spindle motor (not shown) to rotate the polygon mirror 29 at a constant velocity.
Also provided in the laser scanning apparatus 10, is a scanning lens system having first and second f-theta lenses 30, 32 to correct optical errors contained in the deflected laser beams to have a given index of refraction to the optical axis by the deflective reflection surfaces 29a and to refract them to the main scanning direction. First and second reflective mirrors 34, 38 and a cylindrical reflection mirror 36 to reflect the laser beams penetrated through the scanning lens system onto the imaging surface of the photosensitive drum 40 are arranged on the path of the laser beams. The first reflective mirror 34 which does not have a refractive power is adjusted in a direction of arrow B to correct a magnification difference between left and right deterioration in picture quality of the image, and the cylindrical reflection mirror 36, which has a refractive power to the sub-scanning direction, is adjusted in a direction of arrow C to correct skewing of scanning lines in the deteriorating picture quality. Also, the second reflective mirror 38 which finally reflects the laser beams on the imaging surface of the photosensitive drum 40 is formed to mechanically bend and correct bowing of scanning lines.
Operation of the conventional laser scanning apparatus 10 will be explained below.
First, the laser beams emitted from the semiconductor laser 20 pass the collimating lens 22 to be changed into lights parallel to the optical axis. Then, the laser beams passing through the opening part 24 and the cylindrical lens 26 are changed into linear lights having the required size, and are then deflected by the deflective reflection surfaces 29a rotating in a high speed by the spindle motor.
Thereafter, the laser beams are reflected by the first and second reflective mirrors 34, 38 and the cylindrical reflection mirror 36 via the first and second f-theta lenses 30, 32, and then converged as light spots along the main scanning direction on the imaging surface of the photosensitive drum 40.
At this time, since the photosensitive drum 40 is driven to be rotated in the sub-scanning direction by a driving motor (not shown), an electrostatic latent image conforming to the image signal is formed on the photosensitive drum 40 as a result of the scanning movement of the light spots to the main scanning direction and the movement of the photosensitive drum 40 to the sub-scanning direction.
However, in the above described laser scanning apparatus 10, since the second imaging optical system forms a double path by using a plurality of parts including two f-theta lens 30, 32, one cylindrical reflection mirror 36, two reflective mirrors 34, 38, etc., there is a problem that assemblage of the apparatus 10 is difficult and productivity is lowered. Also, according to an increase in a number of parts, adjusting various devices to adjust the mirrors in the scanning apparatus 10 are required, resulting in an increase in fabrication cost.
Further, in the conventional laser scanning apparatus 10 using a front incidence type scanning optical system, since the bow of scanning lines generated as a result of a difference of incident or reflective angles in a sub-scanning cross-section direction at a scanning center part and a scanning periphery part is corrected by the second reflective mirror 38 formed to mechanically bend, accuracy of correction is low. In case of using the conventional laser scanning apparatus 10 as a laser scanning apparatus for color imaging, image stains or spots may be generated due to a difference in the bow quantity of scanning lines between the laser beams. This problem is also presented at the first reflective mirror 34 and the cylindrical reflection mirror 36.
Also, since the conventional laser scanning apparatus 10 images the laser beams in the sub-scanning direction on the image surface by using the cylindrical reflection mirror 36 having the same curvatures to have one refractive power only to the sub-scanning direction, there is a technical limit to realize good optical performance above a 1,200 dpi grade. Particularly, since to obtain a light path from the light deflector 28 to the photosensitive drum 40, size of the cylindrical reflection mirror 36 in a transverse direction (that is, a length thereof in the sub-scanning direction) should be increased, which enlarges the size of the scanning apparatus 10, increasing a weight of the scanning apparatus 10. Thus, when the size and weight of mirrors are enlarged, fabrication and material costs are increased, and at the same time, designing surface shapes of lenses become difficult. Thus, a manufacturing process such as lens designing, mold production, injecting molding, etc., is difficult, resulting in an increase in manufacturing time and cost.